In the dry milling process for manufacturing both food and fuel grades of ethanol from corn and other grains a “beer mash” is made from which the ethanol is removed in a stripper column. The remaining mash is referred to as whole stillage or thick stillage in the fuel ethanol industries and thick slop in the beverage industry. The stillage which is typically 11% to 14% solids and contains all of the other non-starch components of the grains that pass through the process (germ, protein, gluten, hull & fiber etc.). Horizontal dewatering centrifuges are then typically used for removing a portion of the solids from the thick stillage.
The resulting solids, which contain about 65 to 85 percent water, are sent to a drying operation where the solids are dried to less than about 10% percent moisture. The dried solids, referred to as dry distiller grains (DDG's) are used as a nutrient source in animal feed. To manufacture animal feed, it is critical to attain 10% or less moisture because it makes the feed less susceptible to mold and microbial breakdown, which is desirable for long term storage and transport. The energy costs associated with the drying of DDG's can be reduced with more effective dewatering.
The liquid stream from the dewatering device is called centrate (thin stillage), which contains about 6-8 percent solids by weight, about 3 to 4% as suspended solids about 3 to 4% as dissolved solids. A fraction of the centrate or thin stillage is sent back to fermentation and is called backset. Another portion is sent to evaporators for concentration to a syrup which is then blended with DDG's to produce an animal feed referred to as wet feed. Centrate containing high amounts of suspended solids cause efficiency problems (through deposition) in the evaporators, so it is desirable to minimize the amount of solids in the centrate.
The current standard in the industry is the use of high speed horizontal decanter type centrifuges for processing and dewatering of the whole stillage or thick slop. The centrifuges are effective in capturing a portion of the whole stillage stream, which they process. Due to the high shear imparted in the unit a considerable portion of the smaller particles (fines) or the larger particles which are sheared can pass through the unit and are discharged in the centrate. Other drawbacks of centrifuges are the high cost of operation (energy usage) and frequent maintenance and repair.
Therefore, there continues to be an ongoing need for improved dewatering aids and dewatering methods which improve the efficiency of solid-liquid separation in the whole stillage, with concomitant reduction in the energy required to prepare the dry distiller grains